Black liquor chemical recovery unit having an open gas pass



eb- 16, 1955 F. w. HOCHMUTH BLACK LIQUOR CHEMICAL RECOVERY UNIT HAVING AN OPEN GAS PASS 2 Sheets-Sheet 1 Filed Dec. 28, 1962 u n u n a a u FIG.|

INVENTOR. FRANK W. HOCHMUTH Feb. 16, 1965 w, oc u'n- 3,169,512

BLACK LIQUOR CHEMICAL RECOVERY UNIT HAVING AN OPEN GAS PASS 2 Sheets-Sheet 2 Filed Dec. 28, 1962 OOOO-- OOOO FIG. 2

INVENTOR. FRANK W. HOCHMUTH Mu ing 625,.

United States Patent 6) 3,169,512 BLACK LIQUOR CHEMIQAL REQGVERY UNIT HAVING AN SPEN GAS PASS Frank W. Hochmuth, West Simshury, (:112., assignor to Combustion Engineering, 1nd, Windsor, Conn, a corporation of Delaware Filed Dec. 28, 1962, Ser. No. 243,661 2 Claims. (Cl. 122-7) This invention relates to chemical recovery boilers, and in particular to the gas pass construction thereof which will permit the attainment of higher superheat temperatures.

In the sodium base process of paper pulp manufacture, wood chips are cooked in digesters, and the resulting pulp is taken from the digesters, washed and prepared for fur ther use. The so-called black liquor separated from the removed pulp is thereafter passed through evaporators for greater concentration and is then burned in a special furnace to oxidize the organic matter, and to recover the inorganic chemicals. The heat liberated from the burning of the black liquor is utilized by generating and superheating steam.

Problems are encountered in the boiler section of these units because of the large amount of chemical ash particles which is contained in the hot combustion gases exhausted from the furnace. When the temperature of the chemical ash particles carried by the combustion gases is above the fusion or melting temperature, these molten particles tend to separate out on the steam generating and superheating tubes in such a manner that they are rather difficult to thereafter remove. For this reason, from the standpoint of clean boiler operation, it is desirable to maintain the temperature of the combustion gases and the ash particles carried thereby below the fusion or melting point of the ash before these gases encounter any steam generating surface. In recent years, in order to increase the over-all efficiency of these chemical recovery boilers, higher superheat temperatures are now being asked for. It is not uncommon for a chemical recovery boiler today to produce superheater outlet temperatures of 900 F. and above. In order to obtain these higher temperatures, it obviously is necessary to also increase the temperature of the combustion gases in the superheater. New metals which are corrosion resistant and capable of withstanding exposure to high temperatures are available today which make possible the higher operating temperatures of the chemical recovery boilers. However, when the temperature of the combustion gases leaving'the superheater becomes excessive, the chemical ash particles carried by the combustion gases become molten or semi-molten in form. These sticky ash particles when contacting heat exchange surface adhere thereto, causing slagging and plugging of these heat exchangers.

It is an object of this invention to construct a chemical recovery unit which will be capable of producing high superheat temperatures while still-avoiding slagging and plugging in the steam generating section of the boiler because of higher combustion gastemperature.

Additional objects and advantages of the invention will appear from the following description of a preferred embodiment thereof when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a vertical elevation (partly in cross section) of a black liquor recovery system employing the principles of the invention; and

FIG. 2 is a top view of the unit when taken on line 2-2 of FIG. 1.

Referring to FIG. 1, 10 denotes a recovery furnace having a hearth, or smelting zone 12. Black liquor, evaporated to the desired density, is introduced into the furnace 3,159,512 Patented Feb. 16, 1965 "ice by nozzles 18. The air for combustion is supplied by means of duct 16 and air nozzles 14. The black liquor supplied to the furnace is evaporated, so as to increase its solids content to a desired percentage, in cascade evaporator 29. The black liquor is introduced into the evaporator by means of inlet 22 and leaves therefrom through outlet pipe 25. Contained within the evaporator is a rotating drum 23 driven by means of motor 24. This drum in rotating picks up black liquor from the pool contained in the lower portion of the housing, thus exposing it to the hot combustion gases flowing through the upper portion of the housing.

The hot combustion gases generated by the burning of black liquor within the furnace pass into the upper portion of the furnace, flowing first over steam generating tubes 32. Also positioned withinrthe upper portion of the furnace above tubes 32 is final or secondary superheater section 34, and primary superheater tubes 36, 38 and 49.

The combustion gases, after passing over these heating surfaces, leave the upper portion of the furnace through opening 42. The tubes extending through the opening 4-2 are offset with respect to one another so as to allow free passage of gases therethrough. Below the opening 42, the tubes 44 are formed into a single plane, and are finned as shown in FIG. 2, so as to form a solid baflie between the upper furnace and the open gas pass 50. After flowing through this open gas pass 50, the gases make a reverse turn at the bottom thereof, and enter the upwardly extending steam generating portion of the gas pass through opening 52. An ash hopper 53 is positioned beneath the opening 52, so as to permit removal of any ash particles that might settle out at this point as the gases make a turn in direction.

The second portion of the gas pass contains steam generating tubes 32. Pinned tubes 54- form an imperforate bafile between the first and second portions of the gas pass. The tubes 54 passing through the opening 52 are offset and contain no fins, just as the tubes in the opening 42 are constructed. Also, some of the tubes 54 are bent out of the panel and form a part of the tube screen extending into the upper portion of the furnace. The gases leaving the steam generating section of the gas pass flow through opening 56 into the last portion, which contains the economizer tubes 64. A bafide 53 separates these two portions of the gas pass.

Also positioned between these two passages are downcomers 6i) which extend through bafile plates 62. Since it is desired to maintain the water flowing through downcomers 6t relatively cool with respect to the flow of fluid through the steam generating or riser tubes 32, the bafile plates 62 assist in that respect and also prevent the gases from by-passing the economizer tubes 64. If desired, the downcorners could be positioned outside of the gas pass altogether.

The combustion gases flow by means of duct 65 into i the evaporator 26, and are then exhausted to the atmos phere through stack 82. Fan 8% maintains a steady flow of these gases to the atmosphere. Duct 84, the inlet end of which is connected to the stack 82 downstream of fan 80, and the outlet end of which is connected to nozzles es, can be used for recirculating gases back to the open gas pass 50 when desired.

Water enters the economizer tubes 64 by means of inlet header 66, and flows by way of outlet header 68 into the steam and water drum 70. Downcomers 60 supply water to header 74, and also to header 76. The water flows from header 76 into tubes 78 which line all four of the walls of the furnace 11 As seen in FIG. 2, tangent tubes 78 and finned tubes a completely line the outer walls of the furnace and the adjacent gas pass.

The operation of the invention will now be explained. The hot combustion gases laden with chemical dust particles, leaving the upper portion of the furnace, enter open gas pass 58 by means of opening 42. During their downward flow through open gas pass 50, the dust particles, which are generally 100 F. or more above the temperature of the combustion gases, cool down by giving up heat to the combustion gases. Heat is also transferred from the combustion gases to the tubes lining the walls of this pass 50. Also, due to the turbulence of the gases flowing through gas passSt), the temperature of the gases across the gas pass will be equalized. This turbulent flow is caused by the high velocity flow therethrough. Some mixing is also obtained by the introduction of recirculated gases through nozzles 86. Since the recirculated gases will be at a temperature substantially below the gases entering pass 59, they will also tend to lower the over-all temperature within the gas pass 59, thereby promoting the cooling of the chemical dust particles.

From the above it will be seen that higher combustion gas temperatures can be allowed in the superh ater sections, while still preventing excessive combustion gas temperatures and/ or semi-molten chemical dust particles from entering the steam generating section of the boiler. For example, by the use of open gas pass Si), it might be possible to allow the hot combustion gases leaving the superheater section and entering the open gas pass 50 to be as high as 1500 F, with the temperature or" the chemical dust particles carried thereby being somewhat higher. In passing through the open gas pass 59, the temperature of both the combustion gases and the dust particles are reduced and equalized, and thus will enter the steam generating portion of the gas pass at a temperature. where the dust particles are in a solid state. The gases and the dust particles entering the steam generating section should be reduced in temperature to approximately 1100" F. in order to insure the solidification of the dust particles prior to their contact with steam generating tubes. Above this temperature, the particles will become molten or semimolten and sticky in form. These molten particles when coming into contact with heat exchange surface, attach themselves thereto, solidifying thereon. Such ash particles are extremely difiicult to thereafter remove by means of sootblowing or other conventional ash deposit removal equipment, whereas if the dust particles are in the solid state they do not attach themselves to the heat exchanger as readily and also can be easily removed from the heat exchange surface.

. While the preferred embodiment of the invention has been shown and described, it will be understood that minor changes in construction may be made without departing from the spirit and scope of the invention as claimed.

What I claim is:

1. In a system for recovering chemicals and generating and superheating steam from the combustion of black liquor, the combination of a smelter furnace defined by generally vertical walls, means for supplying black liquor into the furnace interior and for burning the same in the lower portion thereof for producing combustion gases laden with chemical ash particles, a superheater heat exchanger including both primary and secondary superheat surface positioned in the flow path of the combustion gases within the upper portion of the furnace, a gas pass connected to the upper portion of the furnace, said gas pass comprised of walls forming a tortuous gas passage made up of a plurality of upwardly and downwardly extending interconnected passages, the walls of the gas passage being lined with fluid cooled tubes, a first portion of the gas passage being connected to the upper portion of the furnace, said first portion being an open gas pass free of any heat exchange members other than the fiuid cooled tubes linin the walls thereof, a second portion of the gas passage being downstream of the first portion and in series flow relationship therewith, said second portion having a plurality of steam generating tubes positioned within the passage over which the combustion gases flow, whereby as the hot combustion gases flow through the open first portion of the gas passage the chemical ash particles give up heat to the combustion gases and the combustion gases give up heat to the fluid cooled tubes, thereby reducing the over-all temperature of the chemical ash particles and the combustion gases prior to entering the second portion.

. in a system for recovering chemicals and generating and superheating steam from the combustion of black liquor, the combination of a smelter furnace defined by generally vertical walls, means for supplying black liquor into the furnace interior and for burning the same in the lower portion thereof for producing combustion gases laden with chemical ash particles, a superheater heat exchanger including both primary and secondary superheat surface positioned in the flow path of the combustion gases within the upper portion of the furnace, a gas pass connected to the upper portion of the furnace, the gas pass comprised of walls forming a gas passage, 9. first portion of the gas passage extending downwardly, and a second portion of the gas passage extending upwardly, fluid cooled tubes lining the walls of the first portion, said first portion being an open gas pass free of any heat exchange members other than the fluid cooled tubes lining the walls thereof, said second portion having a plurality of steam generating tubes positioned within the gas passage over which the combustion gases flow, whereby as the hot combustion gases flow through the open first portion of the gas passage the chemical ash particles give up heat to the combustion gases and the combustion gases give up heat to the fluid cooled tubes, thereby reducing the overall temperature of the chemical ash particles and the combustion gases prior to entering the second portion.

References Cited in the file of this patent UNITED STATES PATENTS 2,196,889 Bailey Apr. 9, 1940 2,213,052 Rosencrants et al Aug. 27, 1940 2,416,462 Wilcoxson Feb. 25, 1947 2,619,411 WilcoXson Nov. 25, 1952 2,869,521 Hardgrove Jan. 20, 1959 2,911,284 Hockmuth Nov. 3, 1959 FOREIGN PATENTS 574,767 Canada Apr. 28, 1959 

1. IN A SYSTEM FOR RECOVERING CHEMICALS AND GENERATING AND SUPERHEATING STEAM FROM THE COMBUSTION OF BLACK LIQUOR, THE COMBINATION OF A SMELTER FURNACE DEFINED BY GENERALLY VERTICAL WALLS, MEANS FOR SUPPLYING BLACK LIQUOR INTO THE FURNACE INTERIOR AND FOR BURNING THE SAME IN THE LOWER PORTION THEREOF FOR PRODUCING COMBUSTION GASES LADEN WITH CHEMICAL ASH PARTICLES, A SUPERHEATER HEAT EXCHANGER INCLUDING BOTH PRIMARY AND SECONDARY SUPERHEAT SURFACE POSITIONED IN THE FLOW PATH OF THE COMBUSTION GASES WITHIN THE UPPER PORTION OF THE FURNACE, A GAS PASS CONNECTED TO THE UPPER PORTION OF THE FURNACE, SAID GAS PASS COMPRISED OF WALLS FORMING A TORTUOUS GAS PASSAGE MADE OF A PLURALITY OF UPWARDLY AND DOWNWARDLY EXTENDING INTERCONNECTED PASSAGES, THE WALLS OF THE GAS PASSAGE BEING LINED WITH FLUID COOLED TUBES, A FIRST PORTION OF THE GAS PASSAGE BEING CONNECTED TO THE UPPER PORTION OF THE FURNACE, SAID FIRST PORTION BEING AN OPEN GAS PASS FREE OF ANY HEAT EXCHANGE MEMBERS OTHER THAN THE FLUID COOLED TUBES LINING THE WALLS THEREOF, A SECOND PORTION OF THE GAS PASSAGE BEING DOWNSTREAM OF THE FIRST PORTION AND IN SERIES FLOW RELATIONSHIP THEREWITH, SAID SECOND PORTION HAVING A PLURALITY OF STEAM GENERATING TUBES POSITIONED WITHIN THE PASSAGE OVER WHICH THE COMBUSTION GASES FLOW, WHEREBY AS THE HOT COMBUSTION GASES FLOW THROUGH THE OPEN FIRST PORTION OF THE GAS PASSAGE THE CHEMICAL ASH PARTICLES GIVE UP HEAT TO THE COMBUSTION GASES AND THE COMBUSTION GASES GIVE UP HEAT TO THE FLUID COOLED TUBES, THEREBY REDUCING THE OVER-ALL TEMPERATURE OF THE CHEMICAL ASH PARTICLES AND THE COMBUSTION GASES PRIOR TO ENTERING THE SECOND PORTION. 